Replaced all line-style "brief" Doxygen comments with block-style.

Coding standard change: Line-style Doxygen comments ("//! ...") are now reserved for commands that apply directly to the code (e.g. "\cond", "\endcond").  For documenting entities within the code, use block-style Doxygen comments ("/*! ... */").
Saved Doxyfile under Doxygen 1.8.3.1.
Removed redundant \brief for enum member.